I have an 8 yr old female beagle, and ever since she woke up this morning she has been jittery, she vomited once (thick, white water substance) and has had 3 dark very watery stools. Her temperature is 100.3

Vomiting and diarrhea can be the result of many different problems (too numerous to list here) so unfortunately the only way of knowing what is wrong with her is to take her to the vet.
If money is short there are a lot of vets willing to work out payment plans for you, especially if you are a long term client of theirs.

Is she drinking OK?
If not buy some pedialyte to giver her to keep her hydrated until you can get her to the vet, diarrhea is very dehydrating.

I agree, you should be taking her to the vet!
Has she eaten anything that she shouldn't have? Large pieces of bones?
Has she been acting weird for some time or is today the only time?
If you are going to bring her in, the vet will more then likely want a stool sample. You can get the little containers either right from your vet, or from a pharmacy. I would try and get a fresh one and bring it in on the initial visit to save time!